1. Field of the Invention
The present invention relates to an electromagnetic flowmeter suitable for measurements of an eccentric flow fluid.
2. Description of the Related Art
It is difficult for an electromagnetic flowmeter to accurately measure a flow rate of an eccentric flow fluid (drifted fluid), as is well-known to those skilled in the art. The eccentric flow fluid is defined as a fluid having a disturbed flow. An eccentric flow is formed in an electromagnetic flowmeter when an upstream straight pipe connected to the electromagnetic flowmeter does not have a sufficient length, when a solid substance or rust is nonuniformly attached to the inner wall surface of the upstream pipe of the electromagnetic flowmeter or a solid substance is precipitated on the bottom of the upstream pipe, or when a fluid to be measured is a slurry containing a solid substance susceptible to precipitation or floating.
In order to accurately measure a flow rate of an eccentric flow fluid, an electromagnetic flowmeter having a function of generating a functional distribution magnetic field is developed. The functional distribution magnetic field is defined as a magnetic field having a magnetic flux density distribution close to a reciprocal number of a value of a weighting coefficient W which is virtually given in a measuring pipe 1, as shown in FIG. 1. Referring to FIG. 1, an angle 2.PHI..sub.B is a spreading angle defined by lines which contact an area for weighting coefficient W=2.0 with respect to the center of the measuring pipe. An angle 2.PHI..sub.R is an angle defined by lines which contact an area for weighting coefficient W=1.2.
Conventional electromagnetic flowmeters each having a function of generating a functional distribution magnetic field are shown in FIGS. 2A and 2B and FIGS. 3A and 3B.
Referring to FIGS. 2A and 2B, a pair of substantially T-shaped yokes 2 are fixed to an outer wall surface of a measuring pipe 1 with small gaps therebetween. Coils 3 are respectively wound around yoke portions 2a extending in the radial direction of the measuring pipe. An outer casing 4 having end flanges 4a is mounted outside the measuring pipe 1. An electrode 6 extends through the wall surface of the measuring pipe 1. A signal from the electrode 6 is extracted outside the casing 4 through a signal line 8. The coils 3 are powered through a line 7.
Referring to FIGS. 3A and 3B, a pair of saddle coils 3 comprising excitation windings are mounted in contact with the outer wall surface of a measuring pipe 1. Other arrangements of the electromagnetic flowmeter shown in FIGS. 3A and 3B ar the same as those in FIGS. 2A and 2B. The same reference numerals as in FIGS. 2A and 2B denote the same parts in FIGS. 3A and 3B, and a detailed description thereof will be omitted.
In order to obtain a functional distribution magnetic field in each conventional electromagnetic flowmeter described above, an angle .PHI. in FIGS. 2A and 3A is set to be a predetermined value. More specifically, the angle .PHI. is about 40.degree. (it varies in accordance with the shape of the coils and configuration of magnetic flux generators) when a length L of the coil is set to be a half of the inner diameter of the measuring pipe, i.e., L=(1/2)D. When the length L is infinite, the angle .PHI. is set to be about 30.degree.. When the length L is smaller than a half of the inner diameter of the measuring pipe, i.e., L&lt;(1/2)D, the angle .PHI. is generally larger than 40.degree..
As described above, the angle .PHI. is relatively large. Gaps between opposing yokes 2 and opposing coils 3 are relatively large and magnetic flux directing opposite directions passes through the gaps. Then, nonsymmetrical magnetic field exists in the gaps. For this reason, fluctuations of a measured signal obtained by the electrodes 6 occurs.
A relationship between the functional distribution magnetic field and the measured signal will be described on the basis of extensive studies made by the present inventor.
In an electromagnetic flowmeter having a relatively small magnetic field length in the fluid flow direction, a relationship between the pair of electrodes 6 and an electromagnetic force generated therebetween is given by equation (1) below: ##EQU1## where D is the inner diameter of the measuring pipe 1, R=r/a, .theta. is the angle used for polar coordinate transformation, Bx is the magnetic flux density at a point P in the x direction, By is the magnetic flux density at the point P in the y direction, Wx is a weighting function representing a magnitude of an electromagnetic force generated between the electrodes at the point P in the x direction, Wy is a weighting function representing a magnitude of an electromotive force generated between the electrodes at the point P in the y direction, and Vz is the flow speed in the measuring pipe 1 in a direction perpendicular to the drawing surface. The Wx and Wy are represented by equations (2) and (3), respectively: EQU Wx=(R.sup.2 sin 2.theta.)/(1-2R.sup.2 cos 2.theta.+R.sup.4)(2) EQU Wy=(1-R.sup.2 cos 2.theta.)/(1-2R.sup.2 cos 2.theta.+R.sup.4)(3)
Since a distance 2h in FIG. 5 is large near the electrodes of the conventional electromagnetic flowmeter using the functional distribution magnetic field, magnetic fluxes having opposite magnetization directions with respect to the y-y line are generated parallel to the y-y line obtained by connecting the electrodes 6 are generated near the electrodes 6, as illustrated in FIGS. 5, 6 and 7. Therefore, the magnetic flux densities Bx and By are complicatedly distributed near the electrodes. As can be understood from equation (1), the magnetic flux densities Bx and By are combined with the weighting functions Wx and Wy to obtain Bx.Wy and By.Wx which influence an electromotive force E. For this reason, when an eccentric flow passes near the electrodes, the measured signal is fluctuated.
In an electromagnetic flowmeter having no function of generating a functional distribution magnetic field, when a fluid to be measured eccentrically flows, a general flow rate measurement error occurs in addition to the fluctuation of the measured signal.